1. Perl Control Flow Learning Objectives:
To understand the commands available for control flow in Perl
To learn some useful operators in Perl

2. “ if “ statement The Perl if statement works almost
the same as in C++:
#!/usr/local/bin/perl5 -w
$user = `whoami`;
chomp($user);
if($user eq "clinton"){
print "Hi Bill!\n";
} … if (-e “datafile”) {…} # if file exists…
The eq operator compares two strings, and returns true if they are equal (use == for numeric comparisons).
The curly braces { } are always required in Perl (even if only one statement inside, unlike C++). This avoids the “dangling else” problem.

3. “ if … elsif … else “ statement
You can also handle a list of cases:
#!/usr/local/bin/perl5 -w
$users = `who | wc -l`;
chomp($users);
if ($users > 4){
print "Heavy load!\n"; }
elsif ($users > 1){
print "Medium load\n";
else {
print "Just me!\n";

4. Relational Operators Perl’s numeric and string comparison operators:
Comparison Numeric String
Equal == eq
Not equal != ne
Less than < lt
Greater than > gt
Less than or equal to <= le
Greater than or equal to >= ge

5. Truth in Perl Truth is flexible in Perl:
Expressions that evaluate to false
0 # traditional false value
"" # the null string
"0" # only non-zero length false string
Some examples of truth:
1 # traditional true value
684 # non-zero numerical values are true
" " # whitespace is true
"hello" # strings are true
"00" # a string

6. And, Or, Not 1 represents true, and 0 false (as in C++).
You can also combine and negate expressions with logical and (&&), logical or (||), and not (!) just like in C++:
#!/usr/local/bin/perl5 -w
chomp($user = `whoami`);
chomp($nme = `who | grep $user | wc -l`);
chomp($nusers = `who | wc -l`);
if($nusers - $nme && $user ne "clinton"){
print "Someone else is logged in!\n"; }
else{
print "All is well!\n";

7. “ while “ statement
The while statement loops indefinitely, while the condition is true, such as a user-controlled condition:
#!/usr/local/bin/perl5 -w
$resp = "no";
while($resp ne "yes"){
print "Wakeup [yes/no]? ";
chomp($resp = <STDIN>); }
$ test11
Wakeup [yes/no]? no
Wakeup [yes/no]? y
Wakeup [yes/no]? yes $

8. Looping using for (1)
for can be used as in C++ to do incrementing loops:
$ cat fac
#!/usr/local/bin/perl5 -w
print "Enter number: ";
$n = <STDIN>;
$fac = 1;
for($i=1; $i<=$n; $i++){
$fac *= $i; }
print "The factorial of $n is $fac\n";
$ fac
Enter number: 5
The factorial of 5
is 120 $
Don’t forget to chomp $n

9. Looping using for (2) With chomp(): $ cat fac
#!/usr/local/bin/perl5 -w
print "Enter number: ";
chomp($n = <STDIN>);
$fac = 1;
for($i=1; $i<=$n; $i++){
$fac *= $i; }
print "The factorial of $n is $fac\n";
$ fac
Enter number: 5
The factorial of 5 is 120 $

10. “last “ command
The last command works like the C++ break command, breaking out of the innermost loop :
$ cat test12
#!/usr/local/bin/perl5 -w
while(1){
print "Wakeup [yes/no]? ";
chomp($resp = <STDIN>);
if($resp eq "yes"){
last; }
$ test12
Wakeup [yes/no]? no
Wakeup [yes/no]? y
Wakeup [yes/no]? yes $

11. String Operators (1)
Concatenate strings with the “.” operator (a period).
$ cat string
#!/usr/local/bin/perl5 -w
$name = "Bill" . "Clinton";
print "$name\n";
print "Bill"."Gates"."\n";
$ string
BillClinton
BillGates $

12. String Operators (2)
The string repetition operator x allows you to repeat a string several times:
$ cat string1
#!/usr/local/bin/perl5 -w
$name = "Bill"x3;
print "$name\n";
$n = 4;
print "Bill" x 2 . "Gates" x $n . "\n";
print 5;
print "\n";
$test = ($n+1) x 4;
print "$test\n";
$ string2
BillBillBill
BillBillGatesGatesGatesGates 5
5555 $

13. Variable Interpolation (1)
Putting variables inside double quotes is called variable interpolation. We have seen many examples of this.
The variable name will be the longest possible variable name that makes sense at that part of the string.
Enclose the variable in a pair of curly braces if needed to override this.

14. Variable Interpolation (2)
$ cat bill1
#!/usr/local/bin/perl5 -w
$bill = "trouble";
$billgates = "cheap";
print "Bill is $bill\n";
print "Bill is $billgates\n";
print "Bill is ${bill}gates\n";
print "Bill is "."$bill\n";
print "Bill is "."$bill"."\n";
$ bill1
Bill is trouble
Bill is cheap
Bill is troublegates $

15. Exponentiation Perl has an exponentiation operator ** unlike C++:
$ cat exp
#!/usr/local/bin/perl5 -w
$n = 2;
$m = 3;
$result = $n ** $m;
print "$n raised to the $m power is $result\n";
$ exp
2 raised to the 3 power is 8 $

16. Operator Precedence
Operator precedence is basically the same as in C++.
As in C++, you can use parentheses to override precedence, and to clarify the grouping.
$ cat prec
#!/usr/local/bin/perl5 -w
$n = 2;
$m = 3;
$result = $n + 1 * $m;
print "$n plus one times $m is $result\n";
$result = ($n + 1) * $m;
$ prec
2 plus one times 3 is 5
2 plus one times 3 is 9

17. Write a Program to compute the Sum
#!/usr/local/bin/perl5 –w
$sum=0;
While (...) {
...